Flame-retardant polyvinyl alcohol base fiber

ABSTRACT

A vinyl-alcohol-based polymer and vinyl-halide-based polymer are dissolved in a common organic solvent for them, a typical example of which is dimethylsulfoxide, to obtain a dope wherein a solution of the vinyl-halide-based polymer having a particle size of 1-50 μm is present in the solution of the vinyl-alcohol-based polymer. This dope is spun into a low temperature solidifying bath comprising a solidifying solvent such as methanol, and the organic solvent. The resultant is subjected to extraction, drying, dry heat drawing, and optional heat shrinking or acetalization to obtain fiber. In the fiber thus obtained, the vinyl-alcohol-based polymer makes sea phases, and the vinyl-halide-based polymer makes island phases whose size is 0.1-3 μm. The crystallinity degree of the vinyl-alcohol-based polymer is 65-85%. 
     The polyvinyl-alcohol-based flame retardant fiber is useful for clothes, industrial materials, living materials and the like. It can be produced at low costs, and has excellent spinning stability and dimensional stability in hot water.

FILED OF THE INVENTION

The present invention relates to a flame retardant fiber of avinyl-alcohol-based polymer (abbreviated to PVA hereinafter), which canbe industrially produced at low costs and is excellent in spinningstability and dimensional stability in hot water, and relates to a fiberwhich can be used suitably for clothes such protective clothes, livingmaterials such as curtains and carpets, industrial materials such as carseats, and the like.

BACKGROUND OF THE INVENTION

As flame retardant fibers, there are known acrylic fibers and polyesterfibers in which flame retardant monomers are copolymerized, rayon fibersin which a flame retardant is kneaded or reacted, thermosetting fibersor aramid fibers whose polymers themselves are flame retardant, cottonor wool that are post-processed with a flame retardant, and the like.However, in acrylic fibers hydrogen cyanide gas is produced when theyare burned. Polyester fibers are melt-dripped. Thermosetting fibers arelow in fiber strength. Aramid fibers are expensive. Cotton and wool haveproblems such as texture-hardening by post-processing, low durabilityagainst washing, and the like. Studies have been made for improvement inthe respective fibers.

On the other hand, PVA-based flame retardant fibers are known in, forexample, Japanese Patent Application Publication Nos. 37-12920 and49-10823. They are used in clothes such as uniforms for fire fightersand working clothes, living materials such as carpets, industrialmaterials such as car seats, and the like. However, they are expensive.In the present situation, quantitative expansion is difficult.

Conventional PVA-based flame retardant fibers are fibers obtained byadding an emulsion of vinyl-chloride-based polymer (abbreviated to PVChereinafter)/water to an aqueous PVA solution and then spinning theresultant dope. PVC, however, is water-insoluble. Consequently, in theconventional method of using water as a dope solvent, it is impossibleto use powdery PVC, which is commercially available, low-priced PVC.Thus, PVC emulsion, which is several times as expensive as the powderyPVC, is used. In order to make PVA-based fibers flame retardant, thisexpensive PVC must be used in an amount of several ten percents of PVA,resulting in high cost of the PVA-based fibers. A mixed aqueous solutionof PVA and PVC emulsion is not stable at 70-100° C. near spinningtemperature, and is especially insufficient in mechanical stability whenthe solution passes through a gear pump. For stabilization, therefore,it is necessary to add a surfactant thereto. This causes higher cost.

Conventional PVA-based flame retardant fibers are produced by mixing PVCemulsion having an emulsion particle size of 0.01-0.08 μm with anaqueous PVA solution; if necessary, adding thereto a tin compound or anantimony compound to obtain a dope; wet spinning the dope into asolidifying bath comprising an aqueous solution of sodium sulfate;subjecting the resultant to drying, dry heat drawing and thermaltreatment; and, if necessary, acetalizing the resultant by formalin forimproving hot water resistance. Moreover, in order to make its strengthhigher, the following method is also performed: a dope wherein boricacid is added to a mixed aqueous solution of PVA and PVC emulsion isextruded into a solidifying bath comprising a mixed aqueous solution ofsodium hydroxide and sodium sulfate, and then the resultant is subjectedto a boric acid-crosslinking process. In any one of these processes,however, because of use of sodium sulfate, which is a dehydrating salt,as the content in a solidifying bath, fine skin layers are formed on thesurface of the fiber immediately after solidification. As a result, itssection becomes a non-uniform skin/core structure. In its core portion,crystallization is liable to become insufficient. In fact, thecrystallinity degree of PVA of this fiber is a small value of 50-60%.Accordingly, there remains room for improving dimension stability,especially dimension stability between dry and wet states even if thefiber is subjected to formalization.

As described above, although conventional PVA-based flame retardantfibers have excellent points compared with other flame retardant fibers,the use thereof is limited because their manufacturing costs are highand their dimension stability is insufficient.

An object of the present invention is to provide a PVA flame retardantfiber which can be industrially produced at low costs and is excellentin spinning stability, and to overcome the drawback that conventionalPVA flame retardant fibers are poor in dimension stability in hot water.

DISCLOSURE OF THE INVENTION

In the light of the situation described above, the inventors eagerlymade studies to produce a PVA-based flame retardant fiber by using acommercially available, inexpensive PVC powder. As a result, theinventors have reached the present invention.

That is, the present invention is a PVA-based flame retardant fibercomprising PVA (1) having a polymerization degree of 1000 or more and asaponification degree of 98 mole % or more, and a halogen-containingvinyl polymer (abbreviated to PVX hereinafter) (2), the fiber being asea and island fiber wherein the polymer (1) is a sea component, and thepolymer (2) is an island component, the size of the island of thepolymer (2) in a cross section of the fiber being from 0.1 to 3 μm, andthe crystallinity degree of the polymer (1) being from 65 to 85%.

Another aspect of the present invention is a method for producing PVAflame retardant fiber comprising: dissolving the polymer (1) and thepolymer (2) into a common solvent for both the polymers; wet spinning ordry-jet wet spinning the resultant dope into a solidifying bath whereina solidifying solvent capable of solidifying polymer (1) and the dopesolvent are mixed; wet drawing the resultant fiber; extracting thesolvent from the fiber; and subjecting the fiber to drying, dry heatdrawing; and optional heat treatment or acetalization, the dope having asea and island structure wherein the solution of the polymer (2) ispresent in an island state in the solution of the polymer (1), and thesize of the diameter of the solution of the polymer (2) being from 1 to50 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a 20000-powered photograph of an example of a sectional shapeof a fiber according to the present invention, which is taken with atransmission electron microscope. FIG. 2 is a 20000-powered photographof an example of a sectional shape of a conventional aqueous PVA-basedflame retardant fiber with which PVC emulsion is mixed, which is takenwith a transmission electron microscope. In FIGS. 1 and 2, a graydispersion component is PVC, and a dispersing medium which is whiterthan the gray component is PVA. A black substance is meta-stannic acid.It was proved by EDX analysis that this black substance is meta-stannicacid. Two centimeters in these figures correspond to actual 1 μM.

BEST MODES FOR CARRYING OUT THE INVENTION

The sea component of the fiber according to the present invention, thatis, a matrix component, is PVA having a polymerization degree of 1000 ormore and a saponification degree of 98 mole % or more. PVA is the onlypolymer widely used that has a solvent common to a solvent for PVX forgiving flame retardation and can form a strong intermolecular hydrogenbond, based on a hydroxyl group, making a highly strong sea/islandstructure.

PVA (1) referred to in the present invention means a polymer havingvinyl alcohol units in an amount of 70% or more by mole of totalconstituent units. Therefore, it is allowable that a monomer describedin the following is copolymerized in an amount of 30% or less by mole:ethylene, itaconic acid, vinylamine, acrylic amide, maleic anhydride, ora sulfonic acid-containing vinyl compound. In order to produce a highlystrong fiber, the saponification degree should be 98% by mole or more.It is preferably 99% by moles or more, and more preferably 99.8% or moreby mole. The upper limit thereof is 100% by mole. Therefore, in PVA (1)a saponificable vinyl unit such as a vinyl acetate unit or a vinylpivalate unit may be copolymerized in an amount of 0-2% by mole of thetotal amount of the vinyl alcohol unit and the saponificable vinyl unit.For the same reason as about the saponification degree, thepolymerization degree of PVA should also be 1000 or more. It ispreferably 1500 or more. It is difficult, however, to industriallyproduce PVA having a polymerization degree of 20000 or more. PVA may beintramolecularly or intermolecularly acetalized, through post-reactionafter fiberization for improving water resistance, by a monoaldehyde, adialdehyde, or a derivative thereof such as formaldehyde, glutalaldhydeor nonandial. Alternatively, PVA may be intramolecularly orintermolecularly crosslinked by other crosslinking agents than thesecompounds.

The island component of the fiber according to the present invention isPVX. Only by using PVX as the island component can the fiber of thepresent invention be made flame retardant. PVX referred to in thepresent invention is a vinyl polymer wherein vinyl units containing ahalogen element, that is, any one of fluorine, chlorine, bromine, oriodine occupy 50-100% by mole of the total constituent vinyl units ofPVX. Examples of PVX include polyvinyl-chloride-based polymer,polyvinylidene-chloride-based polymer, polyvinyl-bromide-based polyer,polyvinyledene-bromide-based polymer, chlorinated polyolefine brominatedpolyolefine and the like. Among these, PVC is preferred from thestandpoint of flame retardation, thermal decomposition resistance,balance with costs, and the like. In PVX, other monomer than the vinylunits may be copolymerized, if flame retardation is not damaged verymuch by copolymerization.

PVX has a low crystallinity and has no fiber producing ability. Even ifPVX is turned into a fiber, the obtained fiber has only a low strength.No PVX fiber has been produced, particularly by wet spinning processes,which are producing processes that are for staple fiber and that areexcellent in cost performance. In the present invention, PVX isincorporated as an island component, so as to cause PVX to play a roleas a functioning component capable of generating hydrogen chloride gaswhen the fiber is exposed to high temperature to be burned and capableof trapping radicals generated in the burning to suppress the burning.

In the present invention, it is preferred to contain PVA (3) having asaponification degree of 50-90 mole % in an amount of 0.1-10% by weightof PVX (2). If a blend solution of the solution of PVA (1) and thesolution of PVX (2) is left as it is, the islands composed of thesolution of PVX (2) are condensed as time passes. As a result, itsspinnability deteriorates so that spinning becomes difficult. On theother hand, in the case that PVA (3) is mixed, the islands composed ofthe solution of PVX (2) are not easily condensed even if the dope isleft as it is. PVX and PVA essentially have bad compatibility with eachother. However, PVA (3) includes a great number of acetic groups so asto have a strong interfacial activity. Thus, PVA (3) has a high affinitywith PVX (2). For this reason, PVA (3) having such a high interfacialactivity functions as an agent for compatibility with PVA (1) and PVX(2) so that the dispersion stability of the islands composed of thesolution of PVX (2) is improved. As PVA functioning as such an agent forcompatibility, PVA having a high interfacial activity is preferred. Forthis purpose, PVA having a low saponification degree is preferred. Asthe saponification is lower, the dispersion stability of the islandcomposed of the PVX solution is increasingly improved. However, if it istoo low, conversely the dispersion stability deteriorates. Thus, thesaponification degree of the PVA (3) is preferably 50-90 mole %, morepreferably 60-88 mole % and most preferably 70-80 mole % Thepolymerization degree of PVA (3) used as the agent for compatibility isnot especially limited. PVA having a polymerization degree of 500 ormore may be used. The polymerization degree is preferably 1700 or more.However, if the polymerization degree is over 20000, it is difficult toproduce PVA industrially.

PVA having a saponification degree of 50-90 mole %, referred to herein,means a polymer having vinyl alcohol units in an amount of 50-90 mole tof the total amount of saponificable units before saponification, andthus has 10-50 mole % of vinyl acetate or vinyl pivalate units, whichare saponificable units. It is also allowable to copolymerize a monomersuch as ethylene, itaconic acid, vinylamine, acrylamide, maleicanhydride, or a sulfonic acid-containing vinyl compound, in an amount of30 mole % or less.

It is preferred that the amount of PVA (1) is 55% or more by weight ofthe total amount of PVA (1) and PVX (2), in order to make PVA (1) havinga polymerization degree of 1000 or more and a saponification degree of98 mole % or more into a sea component and make PVX (2) into an islandcomponent. If the amount of PVA (1) is less than 55% by weight, a partof PVX (2) may become the sea component so that fiber strength islowered or PVX (2) is melted out into an extracting bath. This is notpreferred from the standpoint of performance and processability. On theother hand, if the amount of PVA (1) is more than 95% by weight, theamount of halogen in the fiber is small so that flame retardationbecomes insufficient. Thus, the blend weight ratio of PVA (1)/PVX (2) isfrom 95/5 to 55/45, preferably from 90/10 to 55/45, and most preferablyfrom 80/20 to 60/40, from the standpoint of balance between flameretardation and strength or the like. In the present invention, polymersother than PVA and PVX, various stabilizers, or colorants may be addedif they do not damage the attainment of the object of the presentinvention.

The size of the islands of PVX in the fiber must be 0.1-3 μm. The sizeof the islands of PVX, referred to in the present invention, is anaverage value obtained at the time of subjecting a fiber sample toformalization under a constant length state to make PVA water-insoluble,treating the PVA with epoxy resin to prepare a super-thin slice(thickness: about 800 nm), dyeing the slice with RuO₄ vapor, enlargingand observing a cross section of the resultant super-thin fiber slicewith a transmission electron microscope (referred to as TEM,hereinafter) at 20000 powers, and measuring diameters of 50 islands ofPVX which are arbitrarily chosen from the resultant electron microscopicphotograph.

The most of the island diameters of PVC of conventional PVA-based flameretardant fibers are less than 0.1 μm, and none of the island diametersis 0.2 μm or more. On the other hand, the island diameters of PVX of thePVA-based flame retardant fiber according to the present invention arefrom 0.2 to 1.5 μm. In conventional PVA-based flame retardant fibers,PVC emulsion having a particle diameter of 0.01-0.08 μm is used as rawmaterial PVC. In the producing process thereof, they are drawn at hightemperature so that their diameter becomes narrower. Thus, the size ofthe PVC islands in the resultant fiber does not exceed 0.08 μm, and isgenerally 0.05 μm or less.

On the other hand, in the present invention a common solvent for PVA andan inexpensive PVX powder is used as a dope solvent, and a sea andisland phase separate solution wherein PVA is a sea component and PVX isan island component is used as the dope and is spun into a solidifyingbath. Thereafter, extraction, drying, dry heat drawing, and optionalthermal treatment or the like are performed. In the present invention,the PVX (2) makes islands by separation of the polymer phases of PVA (1)from PVX (2). As a result, in the case that the size of the islands ofthe PVX solution in the dope is from 1 to 50 μm, the island diameter ofPVX in the fiber after the dry heat drawing becomes from 1 to 3 μm. Theparticle size of PVC emulsion used in conventional PVA-based flameretardant fibers, that is, a size of 0.01-0.08 μm, is too small as theisland diameter of PVX in the dope used in the present invention, sothat the dope becomes unstable. Thus, stable spinning is difficult.

Furthermore, in the fiber of the present invention, its polymerizationdegree is 1000 or more, and its crystallinity degree of PVA (1) is65-85%. This is one important characteristic of the fiber of the presentinvention. As described above, conventional PVA-based fibers produced bydehydration and solidification have a section of a skin/core structure,so that their crystallinity degree of PVA is as low as 50-60%. Thus,there is a problem in dry and wet dimensional stability. On the otherhand, the fiber of the present invention is substantially uniformlysolidified by gellation caused by cooling so that its crystallinitydegree is as high as 65-85%. Thus, its strength, and dry and wetdimensional stability are significantly improved compared withconventional fibers.

In the case that the PVA-based fiber of the present invention containsat least one compound selected from the group consisting of tincompounds and antimony compounds in an amount of 0.1-15% by weight ofthe total weight of the polymer, flame retardation is further improvedpreferably. The tin compounds referred to in the present invention arenot especially limited and are allowable if they contain a tin element.Inorganic tin compounds such as tin oxide and meta-stannic acid ispreferred from the standpoint of processability and cost performance.The antimony compounds are not especially limited and are allowable ifthey contain an antimony element. In the same way as about the tincompounds, preferred are oxides such as antimony trioxide and antimonypentoxide. It is presumed that these compounds cause the improvement inflame retardation as follows: they are reacted with hydrogen halide gas,which is produced by the phenomenon that the fiber is exposed to hightemperatures so that PVX is decomposed, so as to yield tin halide orantimony halide, and then these halides trap radicals at the time ofburning to suppress oxidizing reaction; or alternatively theabove-mentioned compounds promote dehydration and carbonization reactionof PVA to suppress burning reaction. The content of tin compound and theantimony compound is more preferably 0.5-10% by weight, and far morepreferably 1-7% by weight from the standpoint of flame retardation andprocessability. The method of dispersing them into the dope is notespecially limited. When PVA and PVX are added to a common solvent anddissolved therein, simultaneously the tin compound or the antimonycompound may also be added thereto.

The following will describe the method for producing the fiber of thepresent invention.

First, PVA and PVX are dissolved into a common solvent to prepare adope. Examples of the common solvent include polar organic solvents suchas dimethylsulfoxide (abbreviated to DMSO hereinafter),dimethylacetoamide and dimethylformamide. DMSO is especially preferredfrom the standpoint of low temperature solubility, low polymerdecomposability and the like. The concentration of the polymer in thedope is preferably within the range of 10-30% by weight.

It is also important that the dope has a phase structure whereinislands, of particles of 1-50 μm, composed of the PVX solution arepresent in the PVA solution. By spinning such a dope, it is possible toobtain a fiber wherein the size of the PVX islands is 0.1-3 μm. Thephase structure of the dope, referred to in the present invention, canbe observed by dropping the dope onto a slide glass so as to be of about200 μm thick, and then taking a photograph thereof with a differentialinterference microscope BX-60 type (manufactured by Olympus Optical Co.,Ltd.). The particle size of the dope, referred to in the presentinvention, is an average value obtained at the time of measuring atleast 50 particles which can be found by the observation with theabove-mentioned differential interference microscope. The case in whichthe majority of the island diameters of the PVX solution are more than50 μm is not preferred in light of processability. Moreover, spinningcannot stably be performed for a long time. If the majority thereof areless than 1 μm, PVA cannot make a clear sea phase. A phase structurehaving an island diameter of 1-40 μm is preferred, and one having thatof 1-30 μm is more preferred.

In the case that the speed of change in the island diameter of PVX is 1μm/hour or more when the dope is allowed to stand still at 80° C., it ispreferred that the dope is continuously stirred from the preparation ofthe dope to spinning. The reason is as follows. PVA and PVX essentiallyhave low compatibility with each other; therefore, if the dope is leftas it is, the PVX islands are condensed by some PVX as time passes.Thus, spinnability deteriorates so that spinning becomes difficult. Thespeed of change in the island diameter is a value obtained by dividingthe difference between average island diameters of PVC just after thefinish of the dissolution of dope and after still stand for 15 hours bya time period for the still stand. This means a strong tendency ofcondensation of the PVC islands.

In the present invention, therefore, the improvement in the dispersionstability of the PVX islands is important, and PVA (3) having asaponification degree of 50-90 mole % makes it possible to removebubbles by the still stand.

Since the presence of PVA (3) having a saponification degree of 50-90mole % contributes greatly to the dispersion stability of the PVX (2)islands in the dope, the method of introduction thereof is alsoimportant. The introduction method includes a manner of adding PVA (3)at the time of suspension polymerization of the PVX (2), and a manner ofadding PVA (3) at the time of dissolving the dope.

In the former manner, even addition of a small amount of PVA (3)contributes to effective dispersion stabilization of PVX (2) since PVA(3) is bonded to PVX (2) at the time of polymerization thereof. However,if the added amount is large, a problem that bubbles become large arisesat the time of washing after the polymerization. Thus, the added amountis preferably within the range of 0.1-3% by weight of thehalogen-containing vinyl monomers.

In the latter manner, a necessary amount of PVA (3) tends to be largerthan in the former manner since PVA (3) cannot be introduced to theinterface between PVA (1) matrixes and PVX (2). However, the addedamount can be large because there does not arise any problem ofbubbling. If the amount is too large, however, the water resistance ofthe resultant fiber is unfavorably lowered. The added amount ispreferably 0.1-10% by weight and more preferably 2-10% by weight of PVX(2).

Both of the former and latter manners may be used at the same time. Thismanner is preferable since the manner makes it possible to suppress theproblems of the respective manners, that is, the bubbling at the time ofthe polymerization and a drop in water resistance. In this case, thetotal amount of PVA (3) can be favorably a small amount within the rangeof 0.1-8% by weight of PVX (2).

The temperature of the dope is preferably 100° C. or lower. If it ishigher than 100° C., the solubility of PVC is improved but itsdecomposition speed remarkably increases so that coloring becomesconspicuous. Its polymerization degree is also lowered. Thus, thetemperature is favorably lower. If it is too low, however, thesolubility of PVC and PVA into the solvent becomes low. Preferably,therefore, the temperature of the dope is 40° C. or higher, and 90° C.or lower. More preferably, it is 50° C. or higher, and 80° C. or lower.Preferably, the viscosity of the dope ranges from 10 to 400 poises inthe case of wet spinning, and ranges from 50 to 2000 poises in the caseof dry-jet wet spinning.

The method for dissolving the polymer is not especially limited. It isallowable to adopt any one of a method of adding the one polymer to asolution wherein the other polymer is dissolved and dissolving theformer polymer therein, a method of dissolving the respective polymersat the same time, a method of mixing respective solutions wherein therespective polymers are independently dissolved into the dope solvent,and the like. It is entirely allowable that acids, antioxidants, or thelike may also be added as stabilizers for the polymer to the dope.

The dope thus obtained is wet spun or dry-jet wet spun into asolidifying bath through spinning nozzles. In the wet spinning process,wherein a solidifying bath contacts spinning nozzles directly, even ifthe pitch of the nozzles is made narrow, spinning can be attained in astate that fibers do not stick to each other. Thus, this process issuitable for spinning of staple fibers, using a multi-hole nozzle. Onthe other hand, in the dry-jet wet spinning process, wherein an air gapis present between a solidifying bath and a spinning nozzle, the stretchof the fiber becomes larger at the air gap. Thus, this process issuitable for high-speed spinning of filament fibers. In the presentinvention, it may be suitably selected, in accordance with purpose oruse, which of the wet spinning process and the dry-jet wet spinningprocess is utilized.

The solidifying bath used in the present invention is a mixed solutionof a solidifying solvent and the dope solvent. The solidifying solventmay be preferably an organic solvent capable of solidifying PVA, forexample, alcohols such as methanol and ethanol; ketones such as acetoneor methyl ethyl ketone; or the like. The weight ratio of the solidifyingsolvent to the dope solvent ranges from 25/75 to 85/15 in thesolidifying bath. The temperature of the solidifying bath is preferably30° C. or lower. It is more preferably 20° C. or lower, and mostpreferably 15° C. or lower from the standpoint of homogenous cooled gel.However, if it is -20° C. or lower, subsequent wet drawing of the fiberbecomes difficult. Thus, it is preferably -20° C. or higher.

Since the fiber of the present invention contains PVX, it is liable tobe colored when it is exposed to high temperature. Its PVA is apt to beoriented and crystallized only by dry heat drawing since solidificationarises uniformly in the cross sectional direction. A fiber wherein PVAis sufficiently oriented and crystallized can be obtained even if, afterthe dry heat drawing, the fiber is not subjected to a higher temperaturethermal treatment as is adopted in ordinary PVA fibers. For this reason,the fiber of the present invention has a few chances that it is exposedto high temperature, so that the coloring of the fiber can besuppressed. Of course, however, in the present invention, dry heattreatment, treatment for formalization or the like may be conducted tofurther improve water resistance.

In the present invention, in order to keep a suitable solidifying level,importance is attached to the composition ratio of the organic solventtype solidifying solvent to the dope solvent in the solidifying bath. Inthe present invention, the ratio (weight ratio)ranges from 25/75 to85/15. If the concentration of the dope solvent is less than 15% byweight, solidifying ability is too high and the fiber is cut at thenozzle so that the spinning condition becomes bad. Additionally, theperformance of the resultant fiber, such as strength and Young'smodulus, is liable to become inferior. On the other hand, if theconcentration of the dope solvent is more than 75% by weight, the fiberis not sufficiently solidified and spinnability is lowered so that thefiber cannot have satisfactory performances such as high strength. Theconcentration of the dope solvent in the solidifying bath is morepreferably from 20 to 70% by weight, and most preferably from 25 to 65%by weight.

In the present invention, for the solidifying bath, the mixed solutionof the solidifying solvent and the dope solvent is used as describedabove. Of course, however, other liquids or solids than the mixedsolution may be dissolved therein if their amount is small. In thepresent invention, the most preferable combination of the solidifyingsolvent with the dope solvent is a combination of methanol with DMSO.

The fiber thread produced in the solidifying bath is forwarded throughwet drawing, extraction of the dope solvent and drying steps, to a dryheat drawing step. In the method of the present invention, a wet drawratio preferably ranges from 1.5 to 5 times. The wet drawn fiber isimmersed into a bath of methanol, ketone or the like, so that the dopesolvent contained in the fiber is extracted and removed. Thereafter, thefiber is dried. Of course, before the drying, an oiling agent or thelike may be given to the fiber. It is necessary to dry-heat-draw thefiber so that a total draw ratio becomes 6 times or more. The dry heatdrawing is usually performed at 180-250° C. The total draw ratio,referred to in the present invention, is a ratio represented by theproduct of a wet draw ratio and a dry heat draw ratio. If the total drawratio is less than 6 times, it is impossible to obtain a fiber havingexcellent strength and Young's module. However, drawing that the totaldraw ratio exceeds 30 times is industrially difficult. The used totaldraw ratio usually ranges from 10 to 20 times.

The following will describe the present invention by way Examples, butthe present invention is not limited to these Examples.

The strength and flame retardant index (LOI value) of fibers in theExamples were measured according to JIS L-1013 and JIS K-7201,respectively.

A boiled water shrinkage ratio (abbreviated to WSr hereinafter) isobtained by applying a hung load of 2 mg/dr to a sample fiber,collecting a predetermined length L₀ (for example, 1.00 m) precisely,boiling the sample under a free condition at 100° C. for 30 minutes, airdrying the sample, applying a hung load of 2 mg/dr again to the sampleafter the air drying, measuring the length (L₁) of the thread precisely,and calculating WSr by the following equation:

    WSr=[(L.sub.0 -L.sub.1)]/L.sub.0 ]×100%

In the Examples, percents (%) and ratios were values based on weight ifnot specified otherwise.

EXAMPLE 1

PVA having a polymerization degree of 1750 and a saponification degreeof 99.8 mole %, a PVC powder having a polymerization degree of 400, andmeta-stannic acid were stirred and dissolved in DMSO at 80° C. under anitrogen gas current for 5 hours to obtain a dope having the followingcomposition: PVA/PVC=65/35, the polymer concentration of (PVA+PVC)=18%,and meta-stannic acid/the polymer=5%. When the dope just after thedissolution was observed with a differential interference microscope, itwas found that the PVC solution made island phases having an islanddiameter (i.e., average particle size) of 25 μm in the PVA solution.However, the speed of change in the island diameter in this dope was alarge value of 2.4 μm/hour. When the dope was left as it was for 15hours to remove bubbles, its spinnability was considerably bad andspinning was impossible. Thus, the same solution as above wascontinuously stirred from the start of dissolution to the end thereof,so that the change in the PVC island diameter was hardly caused. In thisway, stable spinning for a long time became possible. The resultant dopewas wet spun, through nozzles having 2000 holes, each of which had ahole diameter of 0.08 μm, into a solidifying bath at 5° C. wherein theratio of methanol/DMSO was 70/30. While DMSO was extracted by methanol,the resultant fiber was wet drawn into 3.5 times its length. It wasdried by hot air at 100° C. and then was dry-heat-drawn into 4 times at228° C., to obtain a fiber whose single thread thickness was 1.8 denier.Such fiber was continuously produced(spun) for 24 hours. As a result,very stable spinning was implemented without any trouble.

FIG. 1 shows a 20000-powered TEM photograph of a section of theresultant fiber. This photograph demonstrated that the fiber was a seaand island fiber wherein islands of about 0.9 μm size were made of PVC.The LOI value of the present fiber was a high value of 39. Thus, thepresent fiber was a highly flame retardant fiber. The crystallinitydegree of the sea component PVA of the present fiber was a high value of71%, so that its strength was a high value of 8.3 g/d. Furthermore, WSrwas a low value of 2.4%. Thus, the present fiber was excellent indimensional stability against wet. The hue thereof was somewhat yellowand pink, but the coloring thereof was slighter than the coloring ofconventional PVA fibers.

Comparative Example 1

PVC emulsion having a particle size of 0.06 μm, PVA having apolymerization degree of 1750 and a saponification degree of 98.5 mole%, meta-stannic acid and boric acid were stirred and dissolved in waterat 90° C. for 5 hours to obtain a dope having the following composition:PVA/PVC=65/35, the polymer concentration of (PVA+PVC)=20%, meta-stannicacid/the polymer=5%, and boric acid/PVA=2.5%. When this dope wasobserved with a differential interference microscope in the same way asin Example 1, the particles of PVC was too small to be observed. Theresultant dope was wet spun, through nozzles having 2000 holes, each ofwhich had a hole diameter of 0.08 mm, into a solidifying bath at 45° C.which was an aqueous solution containing 20 g/l of sodium hydroxide and350 g/l of soduim sulfate. Next, the fiber was subjected toroller-drawing into 1.5 times, neutralization in a neutralizing bathwhich was an aqueous solution of sulfuric acid and sodium sulfate, wetdrawing into 2.3 times in an aqueous solution of saturated sodiumsulfate at 95° C., washing by boric acid in a washing bath at 30° C.,replacement by sodium sulfate in an aqueous solution of 300 g/l ofsodium sulfate, drying at 100° C., dry heat drawing into 4.0 times at228° C., and dry heat shrinking by 5%, so as to obtain a PVA-based fiberaccording to an aqueous system spinning process.

FIG. 2 shows a 20000-powered TEM photograph of a section of theresultant fiber. From this photograph, the diameter of PVC was about0.05 μm. The LOI value of the present fiber was 39, which was the sameas in Example 1. On the other hand, the crystallinity degree of the seacomponent PVA of the present fiber was a low value of 56%, so that itsstrength was 5.9 g/d. Furthermore, WSr was a high value of 11.5%. Thus,its dimensional stability against wet was insufficient.

The present fiber was treated for formalization reaction with a solutioncontaining 10% formaldehyde and 10% sulfuric acid at 70° C. for 30minutes. The WSr of the resultant fiber was an improved value of 3.5%,but the LOI value was 36, which was lower than that in Example 1. Itsstrength was 5.9 g/d.

EXAMPLE 2

PVA having a polymerization degree of 1750 and a saponification degreeof 99.8 mole %, a PVC powder having a polymerization degree of 400obtained by adding PVA having a polymerization degree of 2400 and asaponification degree of 80 mole %, in an amount of 0.6% of a vinylchloride monomer, into the monomer and polymerizing the mixture, andmeta-stannic acid were added to DMSO. The resultant mixture was thenstirred and dissolved in DMSO at 80° C. under a nitrogen gas current for5 hours to obtain a dope having the following composition:PVA/PVC=67/33, the polymer concentration=18%, and meta-stannic acid/thepolymer=1%. When PVC used herein was analyzed by NMR, PVA having apolymerization degree of 2400 and a saponification degree of 80 mole %was contained in an amount of 0.3% of PVC. When the dope was observedwith a differential interference microscope, it was found that the PVCsolution was present, as an island component having an island diameter(i.e., average particle size) of 11 μm, in the PVA solution. The speedof change in the PVC island diameter was as slow as 0.3 μm/hour. Whenthe dope was left as it was at 80° C. for 15 hours to remove bubbles,its spinnability was not different from that just after the dissolution.Thus, spinnability was very good. The resultant dope was wet spun,through nozzles having 2000 holes, each of which had a hole diameter of0.08 mm, into a solidifying bath at 0° C. wherein the ratio ofmethanol/DMSO was 70/30. Next, the resultant fiber was wet drawn into3.5 times while DMSO was extracted by methanol. Such fiber wascontinuously produced(spun) for 24 hours. As a result, very stablespinning was implemented.

A TEM photograph of a section of the resultant fiber demonstrated thatthe fiber was a sea and island fiber wherein islands of about 0.4 μmsize were made of PVC. The LOI value of the present fiber was a highvalue of 39. The crystallinity degree of the sea component PVA was ahigh value of 70%, so that its strength was an excellent value of 8.6g/d. The hue thereof was substantially the same as in Example 1.

EXAMPLE 3

Dope-dissolution, spinning and drawing were performed in the same manneras in Example 2, except addition of PVA having a polymerization degreeof 1750 and a saponification degree of 99.8 mole % and PVC polymerizedwithout any addition of PVA and having a polymerization degree of 400 ata PVA/PVC ratio of 67/33, and addition of PVA having a polymerizationdegree of 2400 and a saponification degree of 80 mole % in an amount of0.6% of PVC. When the dope was observed with a differential interferencemicroscope, it was found that the PVC solution was present, as an islandcomponent having an island diameter (i. e., average particle size) of 18μm, in the PVA solution. The speed of change in the PVC island diameterwas 0.5 μm/hour, which was faster than that in Example 2. However, whenthe dope was left as it was at 80° C. for 15 hours to remove bubbles,its spinnability was hardly different from that just after thedissolution. Thus, spinnability was very good in the same way as inExample 2.

A TEM photograph of a section of the resultant fiber demonstrated thatthe fiber was a sea and island fiber wherein islands of about 0.5 μm insize were made of PVC. The LOI value of the present fiber was a highvalue of 38. The crystallinity degree of the sea component PVA was ahigh value of 71%, so that its strength and WSr were excellent values of8.3 g/d and 2.5%, respectively. The hue thereof was substantially thesame as in Example 1.

EXAMPLE 4

Dope-dissolution, spinning and drawing were performed in the same manneras in Example 2, except addition of PVA having a polymerization degreeof 1750 and a saponification degree of 99.8 mole % and PVC which wascopolymerized with 5% of vinyl acetate and 2.5% of hydroxypropylacrylate and without any addition of PVA and which had a polymerizationdegree of 400 at a PVA/PVC ratio of 67/33, and addition of PVA having apolymerization degree of 2400 and a saponification degree of 80 mole %in an amount of 0.5% of PVC. When the dope was observed with adifferential interference microscope, it was found that the PVC solutionwas present, as an island component having an island diameter (i.e.,average particle size) of 10 μm, in the PVA solution. The speed ofchange in the PVC island diameter was as slow as 0.3 μm/hour. Even whenthe dope was left as it was at 80° C. for 15 hours to remove bubbles,its spinnability was not different from that just after the dissolution.Thus, spinnability was very good in the same way as in Example 2.

A TEM photograph of a section of the resultant fiber demonstrated thatthe fiber was a sea and island fiber wherein islands of about 0.4 μm insize were made of PVC. The crystallinity degree of the sea component PVAwas a high value of 70%, so that its strength and WSr were excellentvalues of 8.4 g/d and 2.7%, respectively. The LOI value was a somewhatlow value of 37 because PVC was a copolymer. On the other hand, the huethereof was better than that in Example 1.

EXAMPLE 5

PVA having a polymerization degree of 1750 and a saponification degreeof 99.8 mole % was added to a suspension liquid of meta-stannic acid andantimony trioxide in DMSO and then the suspension liquid was stirred anddissolved at 80° C. under a nitrogen gas current for 5 hours to obtainthe following solution: PVA=20%, meta-stannic acid/PVA=6%, and antimonytrioxide/PVA=1.5%. In another dissolving machine, PVA having apolymerization degree of 2400 and a saponification degree of 80 mole %was added, in an amount of 0.5% of PVC powder having a polymerizationdegree of 400, into the PVC powder. This was stirred and dissolved intoDMSO at 70° C. under a nitrogen gas current for 5 hours, to obtain a 20%PVC solution. The resultant PVA solution and PVC solution were mixedwhile being weighed by gear pumps. The mixture was stirred and mixed at3000 rpm by T.K. pipeline homomixer (manufactured by TOKUSHU KIKA KOGYOCO., LTD.) in the middle of its pipe. As for the mixed solution, theratio of PVA/PVC was 67/33, a total polymer concentration was 20%, theamount of meta-stannic acid was 4% of the polymer, and the amount ofantimony trioxide was 1% of the polymer. When the dope was observed witha differential interference microscope, it was found that the dope had aphase structure wherein PVC solution made island phases having an islanddiameter (i.e., average particle size) of 37 μm, in the PVA solution.This mixture dope was subjected to spinning, wet drawing, extraction,drying, heat drawing in the same way as in Example 1, and was furthersubjected to dry heat shrinking treatment by 5% at 230° C.

A TEM photograph of a section of the resultant fiber demonstrated thatthe fiber was a sea and island fiber wherein islands of about 1.4 μm insize were made of PVC. The hue of the present fiber was superior to thefiber of Example 1. Its LOI value was 37. The crystallinity degree ofthe sea component PVA was 70%. Its strength and WSr were 7.6 g/d and2.0%, respectively. The process in this Example was continuouslyconducted for 24 hours, so that a fiber was produced with good spinningstability.

Comparative Example 2

PVA-based flame retardant fiber was produced in the same manner as inExample 5, except that PVA having a polymerization degree of 2400 and asaponification degree of 80 mole % was not added to PVC powder. When thedope was observed with a differential interference microscope, it wasfound that the PVC solution made island phases having an island diameter(i.e., average particle size) of 70 μm, in the PVA solution. In the sameway as in Example 5, continuous spinning was conducted. For up to 3hours, no problems occurred, but after that, spinnability deteriorated.After 6 hours passed, spinning was unavoidably stopped.

Industrial applicability

The fiber of the present invention is an invention for improving afurther cost performance of PVA-based flame retardant fibers which areexcellent in burning gas toxicity, resistance against melt drip,strength, costs, durability against washing, texture and the likecompared with fibers other than the PVA-based fibers, such as flameretardant acrylic fibers, flame retardant polyester fibers,thermosetting fibers, aramid fibers, flame retardant cotton, flameretardant wool and the like. In conventional PVA-based flame retardantfibers, a special, expensive PVC emulsion solution is used as PVX forobtaining flame retardation, and a dope mixed with an aqueous PVAsolution is spun into an aqueous solution containing sodium sulfate.Subsequently, drawing, thermal treatment and formalization areperformed. The fiber of the present invention, however, is obtained asfollows. Commercially available, inexpensive PVX powder is used as PVCand dissolved in a common solvent for PVX and PVA to prepare, as a dope,a mixture solution having a phase structure wherein a PVX solution makesisland phases having a specific size in the PVA solution. This dope issubjected to cooled gel spinning in a solidifying bath, at lowtemperature, comprising a solidifying solution and the dope solvent;drawing; and optional thermal treatment and acetalization. The fiberthus obtained has a high crystallinity degree of 65-85%. This isdifferent from conventional PVA-based fibers, which have a lowcrystallinity degree of 50-60%. The fiber of the present invention isalso very good in spinning stability. Therefore, PVX powder, which isseveral times as low-priced as PVC emulsion, can be used as PVX thatmust be used in a large amount of several ten %. In addition, the phaseof PVA can be highly oriented and crystallized to obtain a PVA-basedflame retardant fiber having high cost performance. The fiber of thepresent invention can be effectively used in fields concerned withprotective clothes such as combat uniforms and uniforms for firemen,industrial materials such as car sheets, vehicle spring receivers andair filters, and living materials such as curtains, carpets, blankets,bedclothes, sheet covers, and inner cotton.

What is claimed is:
 1. A polyvinyl-alcohol-based flame retardant fibercomprising a vinyl-alcohol-based polymer (1) having a polymerizationdegree of 1000 or more and a saponification degree of 98 mole % or more,and a halogen-containing vinyl polymer (2),the fiber being a sea andisland fiber wherein the vinyl-alcohol-based alcohol (1) is a seacomponent, and the halogen-containing vinyl polymer (2) is an islandcomponent, the size of the island of the halogen-containing vinylpolymer (2) in a cross section of the fiber being from 0.1 to 3 μm, andthe crystallinity degree of the vinyl-alcohol-based polymer (1) beingfrom 65 to 85%.
 2. The fiber according to claim 1, which furthercomprises a vinyl-alcohol-based polymer (3) having a saponificationdegree of 50-90 mole % in an amount of 0.1-10% by weight of thehalogen-containing vinyl polymer (2).
 3. The fiber according to claim 1,which further comprises at least one compound selected from the groupconsisting of tin compounds and antimony compounds in an amount of0.1-15% by weight of the total polymer weight.
 4. A method for producinga polyvinyl-alcohol-based flame retardant fiber comprising: dissolving avinyl-alcohol-based polymer (1) having a polymerization degree of 1000or more and a saponification degree of 98 mole % or more, and ahalogen-containing vinyl polymer (2) into a common solvent for both thepolymers; wet spinning or dry-jet wet spinning the resultant dope into asolidifying bath wherein a solidifying solvent capable of solidifyingthe vinyl-alcohol-based polymer (1) and the dope solvent are mixed; wetdrawing the resultant fiber; extracting the solvent from the fiber; andsubjecting the fiber to drying, dry heat drawing, and optional heattreatment or acetalization, the dope having a sea and island structurewherein the solution of the halogen-containing vinyl polymer (2) ispresent in an island state in the solution of the vinyl-alcohol-basedpolymer (1), andthe size of the diameter of the solution of thehalogen-containing vinyl polymer (2) being from 1 to 50 μm.
 5. Themethod according to claim 4, wherein the dope is continuously stirredduring the period from the production of the dope to spinning in thecase in which the speed of change in the island diameter of thehalogen-containing vinyl polymer (2) in the dope is 1 μm/hour or higher.6. The method according to claim 4, wherein the dope is obtained bydissolving the vinyl-alcohol-based polymer (1), the halogen-containingvinyl polymer (2) and the vinyl-alcohol-based polymer (3) into a commonsolvent for the polymers (1), (2) and (3) so that the amount of thepolymer (3) is from 0.1 to 10% by weight of the polymer (2).
 7. Themethod according to claim 4, wherein as at least one part of thepolymers (2) and (3), the polymer (2) containing the polymer (3) is usedwhich is obtained by adding the polymer (3) in an amount of 0.1-3% byweight of the halogen-contained vinyl monomer at the time ofpolymerization of said monomer.
 8. The method according to claim 4,wherein as a part of the polymer (3), the polymer (2) containing thepolymer (3) is used and the remaining of the polymer (3) is added to thedope at the time of preparation of the dope, so that the amount of thepolymer (3) in the dope is from 0.1 to 8% by weight of the total polymerweight in the dope.
 9. The method according to claim 4, wherein at leastone compound selected from the group consisting of tin compounds andantimony compounds is mixed with the dope, in an amount of 0.1-15% byweight of the total polymer weight.